1. Field of the Invention
This invention relates generally to aluminum nitride (AlN) and more particularly, to epitaxial cubic (zinc-blende) AlN films that may have a thickness on the order of 1000 Å or greater and a method of making same by plasma source molecular beam epitaxy (PSMBE).
2. Description of the Related Art
The Group III-V nitride semiconductors (GaN, AlN, and InN) are of great interest for their potential as optoelectronic materials. These materials have an equilibrium crystal structure which is wurtzite, or hexagonal. The bandgaps of the wurtzite nitride semiconductors are all direct and their alloys have a continuous range of direct bandgaps values ranging from 1.9 eV for InN to 4.0 eV for GaN to 6.2 eV for AlN. As optical materials, these semiconductors are active from the orange into the ultraviolet.
Formation of nitride semiconductors for device applications requires, among other things, achieving the correct stoichiometry, inducing the correct energy to form a highly crystalline matrix, maintaining high purity, and matching the lattice parameters of the semiconductor and the substrate. Much effort was expended in the 1960's and 1970's to grow and characterize Group III-V nitride semiconductors. However, the effort was ineffectual to achieve high-quality material. Recently, there has been renewed effort to create higher quality Group III-V nitride semiconductors. However, GaN, AlN, and InN produced by conventional methods have high n-type background carrier concentrations resulting from native defects commonly thought to be nitrogen vacancies. Nitrogen vacancies affect the electrical and optical properties of the film. Oxygen contamination is also a major problem. Thin layers of AlN have been prepared by magnetron sputtering, chemical vapor deposition, ion beam sputtering, and ion beam assisted deposition.
However, these methods operate at elevated temperatures and generally do not result in epitaxial growth (i.e., growth oriented in one direction). Moreover, while these techniques have been successful in producing polycrystalline AlN films, they have not been successful in producing electronic-grade single crystal films.
AlN, in particular, is a promising material for high-power, high-temperature optoelectronic devices since it has very high chemical and thermal stability, good thermal conductivity, and fast Rayleigh velocity. AlN crystallizes, under normal conditions, into the thermodynamically stable hexagonal wurtzite structure. However, the metastable cubic zinc-blende structure is expected to be easier to dope and to have decreased phonon scattering, and therefore, to have higher ballistic electron velocities, thermal conductivity, and acoustic velocities due to its higher symmetry. These properties give rise to many exciting potential device applications.
There have been several reports of AlN having the metastable cubic zinc-blende structure. These reports, however, lack detail on the physical, electrical, and optical properties of cubic zinc-blende AlN because the films were too thin for such studies, and certainly too thin to be useful for optoelectronic devices which require thicknesses on the order of at least 2000 Å, and preferably 4000 Å to 8000 Å. The lattice constant of zinc-blende AlN was calculated theoretically to be 4.38 Å using data from the elastic constants of wurtzite AlN. This value was later confirmed experimentally on a 12 nm thick film of zinc-blende AlN grown pseudomorphically on cubic TiN sandwiched between a tetragonal Al3Ti overlayer. To date, however, there have been no reports of successful fabrication of thick, device-quality films of zinc-blende AN. The known AlN films have been mixed hexagonal and possibly cubic (which could be the rock salt structure).
It is, therefore, an object of the invention to prepare zinc-blende AlN of sufficient quality and thickness to characterize it for its mechanical, optical, and electrical properties and to be useful for device fabrication.
It is also an object of the invention to prepare device quality, single crystal, epitaxial films of cubic zinc-blende AlN.
It is a further object of the invention to produce a semiconductor devices that include an epitaxial film(s) of single crystal zinc-blende AlN.
It is an additional object of this invention to provide a method of making an epitaxial film of zinc-blende AlN.